This invention relates to a battery capacity calculating method for calculating the discharging capacity or the residual capacity of a battery.
This application claims priority of Japanese Patent Application No. 2002-139167 filed in Japan on May 14, 2002, the entirety of which is incorporated by reference herein.
As the methods for calculating the capacity of a battery, for example, a lithium ion secondary battery, a method for calculating the residual capacity by a current integrating method of estimating the capacity from the value of Coulombs, and a method of calculating the residual capacity based on the voltage across battery terminals, have so far been used.
In calculating the battery capacity, there are produced variations in the value of the Coulomb or in the terminal voltage due to the difference in measurement conditions, or in the state of deterioration, even with the battery of the same type, so that it is extremely difficult to predict the current capacity or residual capacity of the battery with the above-described pre-existing capacity calculating method. This will be explained below by referring to experimental results.
A cylindrically-shaped lithium ion secondary battery, with a diameter of 18 mm and a length of 65 mm, was provided, and was electrically charged by a constant current constant voltage charging method, as a recommended charge method. That is, a constant current of 1 A is supplied to the battery and, when the voltage of 4.2V is reached, the charging state is switched to the constant voltage charge to maintain the voltage at 4.2 V for three hours. The nominal voltage of this battery under this charging condition is 1.7 Ah.
The battery of the initial state, fully charged by the above-described charging, was discharged to 2.5V, under the operating environment of the room temperature of 20° C. and the discharging current values of 0.1 A, 0.2 A, 0.4 A, 0.8 A, 1 A and 2 A and a discharge curve was measured to find the discharging capacity during this time interval. FIG. 30 shows a discharge curve of the battery of the initial state, that is during its initial charging/discharging cycle. It is noted that, for simplifying the explanation, the residual capacity, obtained on subtracting the discharging capacity, as found from the full charging capacity, is plotted on the abscissa. Additionally, the battery was deteriorated by a cyclic test of repeatedly carrying out the charging and discharging at 20° C. and discharging curves were measured of the battery at the 100th, 300th and 500th cycles to find the values of the discharging capacity.
The charging capacity at each current value, with the discharging capacity at each current value, with the 1 A discharging of 100%, is shown in the following Table 1. The discharging capacity ratio at each charging/discharge cycle is also shown in Table 1.
TABLE 1numberof cycles0.1 A0.2 A0.4 A0.8 A1.0 A2.0 A1discharging1.70881.70731.70751.70491.70411.6885capacity Ahrdischarging100.099.999.999.899.798.8capacity ratio %100discharging1.58811.58031.58011.57661.57721.5621capacity Ahrdischarging100.099.599.599.399.398.4capacity ratio %300discharging1.47071.45281.44611.43731.43391.4072capacity Ahrdischarging100.098.898.397.797.595.7capacity ratio %500discharging1.15361.10751.10181.08871.08471.0421capacity Ahrdischarging100.096.095.594.494.090.3capacity ratio %
As may be seen from the above Table 1, the discharging capacity is decreased with the increasing discharge current. This tendency is the more pronounced the severer the deteriorated state of the battery. Thus, even when the capacity of each fully charged battery is found, the Coulomb efficiency (discharging capacity/charging capacity) is changed with the discharging current, so that, with the method of calculating the residual capacity by the current integrating method of estimating the residual capacity from the Coulomb quantity, as the pre-existing residual capacity detection method, the difference between the estimated residual capacity and the actual residual capacity is increased to render it difficult to estimate the residual capacity to a high accuracy.
Moreover, as may be seen from FIG. 30, the discharge curve undergoes voltage decrease on the whole, with the increasing discharging current, and has plural voltages for a given residual capacity. Stated differently, the discharge curve is changed with the discharging conditions, so that, with the preexisting voltage method of simply measuring the discharge voltage to find the residual capacity based on a discharge curve operating as a reference, there is a fear that he estimated residual capacity is deviated significantly from the actual residual capacity.